The processes used in water treatment depend largely on the nature of the raw water. Water supplies which feed industrial plants for the production of potable water for distribution and consumption often contain unacceptably high levels of dissolved, dispersed or suspended organic compounds and materials. Most organic compounds and materials found in raw water supplies are natural organic matter (“NOM”). A fraction of the NOM in the raw water supply is represented by dissolved organic compounds which present particular difficulties. These organic compounds referred to as dissolved organic carbon (“DOC”) are one of the main causes of water discolouration. DOC often includes compounds such as humic and fulvic acids which are water soluble at certain water pH levels. Humic and fulvic acids are not discrete organic compounds but mixtures of organic compounds formed by the degradation of plant residues.
The removal of DOC from water and especially the humic and fulvic acids is necessary to provide high quality water suitable for distribution and consumption. A majority of the compounds and materials which constitute DOC are soluble and not readily separable from the water. The DOC present in raw water renders conventional treatment difficult and expensive.
The production of safe potable water from a raw water supply often requires treatment of the raw water to make it commercially acceptable, as well as safe to drink. The removal of suspended matter and DOC is an important aspect of this treatment. Two approaches are commonly used for the removal of suspended matter and DOC. One involves coagulation and the other membrane filtration.
In the process involving coagulation, a coagulant is applied to destabilise and combine with suspended matter and DOC so that they coalesce and form a floc, which can then be physically removed by methods such as floating, settling, filtration or a combination thereof. Coagulants such as alum (aluminium sulphate), various iron salts and synthetic polymers are commonly used in processes for water treatment. However, many raw water sources have high levels of DOC present, which reacts with the coagulant requiring a higher coagulant dose than would be required for removal of suspended matter alone. The bulk of the floc formed may then be removed by sedimentation or flotation and the water containing the remainder of the floc passed through a filter for final clarification. However, even after such treatment, the treated water may contain as much as 30-70% of the initial DOC.
In the membrane filtration process the water is filtered through a membrane. There are four commonly available membrane processes currently in use for water treatment. Microfiltration (“MF”) and Ultrafiltration (“UF”) are two processes generally used to remove turbidity and solid particles from water. However, if the water contains high levels of DOC then the membranes used in MF and UF tend to be fouled by the DOC, thereby reducing the flux across the membrane, reducing the life of the membranes and increasing operating costs. The two other membrane processes, Nanofiltration (“NF”) and Reverse Osmosis (“RO”) are typically used to remove low molecular weight compounds from water, including DOC, to allow its use as potable water. These membrane systems are also used in desalination of seawater and brackish waters (e.g. demineralisation). These membrane systems are designed to handle water containing high levels of DOC but have much higher capital and operating costs than MF and UF when used in the production of potable water.
Ion-exchange resins can also be used for removing DOC present in raw water. Ion-exchange techniques conventionally involve passing water through a packed bed or column of ion-exchange resin. Target species such as DOC are removed by being adsorbed onto the ion-exchange resin. Ion-exchange resins have been used to remove up to 90% of the DOC in raw water.
Ion-exchange resins may also be used in conjunction with other methods of water purification including those mentioned previously. Sufficient resin may be added to remove a percentage of the DOC such that the cost of any subsequent treatment used to meet water quality objectives is minimised. For example, the use of ion-exchange resin for the removal of DOC can facilitate the reduction of the amount of coagulant required to achieve acceptable product water quality. Ion-exchange resin may also aid in significantly reducing the capital and operating costs of membrane filtration.
In order to minimise costs in water processing the ion-exchange resins should be recyclable and regenerable. Recyclable resins can be used multiple times without regeneration and continue to be effective in adsorbing DOC. Regenerable resins are capable of being treated to remove adsorbed DOC, and as such, these regenerated resins can be reintroduced into the treatment process.
Ion-exchange resins incorporating dispersed magnetic particles (magnetic ion-exchange resins) readily agglomerate due to the magnetic attractive forces between them. This property renders them particularly useful as recyclable resins as the agglomerated particles tend to settle quickly and are therefore more readily removable from the water. A particularly useful magnetic ion-exchange resin for the treatment of raw water is described in WO96/07675, the entire contents of which is incorporated herein by reference. The resin disclosed in this document has magnetic particles dispersed throughout the polymeric beads such that even when the resin become worn through repeated use, the worn resin retains the magnetic character. Ion exchange beads of the type disclosed in this document are available from Orica Australia Pty. Ltd., under the trademark, MIEX®. One of the benefits of using MIEX® resin is that its small size allows the resin to be pumped.
WO 96/07615, the entire contents of which is incorporated herein by reference, describes a process for removing DOC from water using an ion-exchange resin which can be recycled and regenerated. This process is particularly useful in treating raw water with magnetic ion-exchange resin of the type described in WO96/07675.
The preferred ion-exchange resins disclosed in WO96/07675 are magnetic ion-exchange resins which have, throughout their structure, cationic functional groups which provide suitable sites for the adsorption of DOC. These cationic functional groups possess negatively charged counter-ions which are capable of exchanging with the negatively charged DOC. The negatively charged DOC is removed from the raw water through exchange with the resin's negative counter ion. As a result of this process DOC becomes bound to the magnetic ion-exchange resin and the function of the ion-exchange resin is reduced. Such resins can be referred to as used, spent or loaded resins. When producing potable water for distribution and consumption it is particularly important to be able to regenerate the loaded resin in an efficient and cost-effective manner. This can also be important when using ion-exchange resins for other purposes such as removal of contaminants in potable or waste water treatment and metal recovery.
WO 96/07615 discloses a process for regenerating magnetic ion-exchange resin by contacting it with brine (which is substantially a NaCl solution). The brine solution in such a process is the “regenerant”. The resin is regenerated by the exchange of a chloride ion for a DOC ion from the loaded resin. The byproduct from regeneration is referred to as the “spent regenerant” and is primarily a mixture of the removed DOC and brine. The spent regenerant from a regeneration process is discharged into the ocean or may be used as land fill.
The regeneration process disclosed in WO 96/07615 involves passing brine through a packed column of loaded resin. The regeneration can also be affected by a mixing or agitation process. In practice, these regeneration processes are performed in large batch wise operations. For example, loaded MIEX® resin is removed from the treatment process from the settler underflow and generally transferred to one of two large regeneration vessels. When one vessel is filled the settler underflow stream is directed to the second regeneration vessel while the one that has been filled undergoes regeneration. The regeneration is performed either in:    (i) a mixing tank where a mechanical agitator mixes the regenerant solution with the resin (agitated tank regeneration), or    (ii) a tank where the regenerant solution is passed through a stationary bed of resin with the ion exchange occurring while the regenerant is in contact with the resin (column regeneration or plug flow regeneration).
In contrast, fixed bed filtering systems retain the resin within large treatment vessels and the resin is regenerated by taking the treatment vessel offline and washing the loaded resin with a suitable regenerant to regenerate the resin.
Each step in the regeneration process (including rinsing steps if required) can take a long time to complete. Large regeneration tanks may need to be filed with resin and regenerant, then allowed time to regenerate the resin, then the regenerant needs to be drained and the resin washed before recycling the regenerated resin. Large qualities of regenerant, typically brine, are used and result in large quantities of spent regenerant containing the compounds such as DOC and other electrolytes that were bound to the ion exchange resin together with electrolytes ordinarily present in the regenerant. Large volumes are involved as the regenerant needs to be used in a dilute solution because ion exchange resins suffer from osmotic shock which damages the resin reducing its effectiveness. To avoid this saturated regenerant precursor solutions are normally diluted before use as regenerant and therefore increases the total volume of spent regenerant. The washing step also produces a waste stream containing dilute concentrations of electrolytes ordinarily present in the regenerant. The spent regenerant and wash waste stream may need to be collected, processed, treated or concentrated and removed by tanker for disposal. At many sites it is not environmentally acceptable to send large qualities of brine into the sewers/drain system. This increases the overall capital and maintenance costs due to the additional required equipment for managing the dilution of the regenerant precursor solution, pumps, storage vessels and associated sensors and other equipment.
It will also be appreciated that the treatment plant requires a large inventory of resin to cover both the resin in use in the water treatment process and also the resin being regenerated and spare resin to replace operating losses. This resin will need to be stored on site. This can create further difficulties when the plant is shut down for maintenance. All the resin needs to be stored in containment vessels whilst the water treatment tank is subjected to maintenance.
The additional systems required for on site regeneration of loaded resin are a significant problem and may prevent the use of resin systems at some water treatment sites. In addition to the storage requirements for regenerant and spent regenerant, the overall regeneration systems can have a large footprint as the amount of loaded resin which can be regenerated at any one time has traditionally been limited by the size of the regeneration containment vessel. Large vessels have a large footprint and the required space may not be available at the treatment plant, limiting the regeneration process to vessels of restricted sizes or requiring the loaded resin to be processed off site. Furthermore, the cost associated with manufacturing the large containment vessel(s) often require specialised design, engineering, manufacture and equipment which can also significantly increase the initial installation costs and time particularly when the customer has little expertise with on site resin regeneration.
Accordingly, there is a need for alternative ion-exchange regeneration processes, to assist in addressing one or more of the shortcomings of the currently available regeneration processes and thereby allow the increased use of resin systems such as those involving MIEX® resin.